This invention relates to a method and apparatus for treating a video signal and decoding the same. The invention has particular utility in the treatment of video signals and the provision of video tapes for normal consumption by way of sale or hire having a treated video signal thereon which limits the possibility of making an acceptable copy of the treated video signal on to another video tape using a standard video signal recording system (VCR), having an automatic gain control (AGC) for adjusting the video signal level during recording thereof, whilst allowing for normal playing and viewing of the videotape having the original treated signal thereon. The invention also has utility with respect to the decoding of such treated video signals to allow reconstitution of the original video signal in certain situations where this may be necessary.
In a standard phase alternating line (PAL) video signal, there are 312 and one half lines per field, where by two fields are displayed in succession in an interfaced manner to generate one complete picture. Consequently, there are 625 separate raster lines on a screen making up one complete picture. For the purposes of the present invention, a frame is defined to constitute a single field, two frames being required to be displayed one after the other in an interlaced manner to make up a complete picture.
Methods and apparatuses for treating a video signal to prevent unauthorised recordings thereof have been described in the past. For example, U.S. Pat. No. 4,163,253 by Sony describes a method and an apparatus for treating a video signal to prevent satisfactory video picture reproduction as a result of further recording the treated video signal using a VCR having an AGC used in the recording process. The method employed by the Sony patent relies upon inserting a pulse signal into the back porch of the horizontal synchronisation pulse having an amplitude that is much greater than the peak white level of a VCR on which it may be recorded, and which pulse signal is used to upset the normal operation of the AGC of the VCR recording the treated signal. The specification provides a good overview of the conceptual operation of AGCs used during the recording operation of normal VCRs, which is relevant to the present invention.
Notwithstanding the method and apparatus described in the Sony U.S. Pat. No. 4,163,253, the described arrangement did not work for reasons outlined in U.S. Pat. No. 4,819,098 by Macrovision. Moreover, as described in the latter patent specification, many television monitors and receivers use the back portion region of the back porch, prior to the picture information portion of the video signal, for black-level clamping. Thus, although the basic principle may have worked in preventing copying of the treated signal by VCRs with AGCs, the treated signal also could not be properly played and viewed on a VCR when the AGC was not being used at all, since the inserted pulse signal interfered with the black-level clamping function of monitors and receivers making the picture black.
Macrovision in their U.S. Pat. No. 4,819,098 looked at using the same principle of upsetting the operation of the AGC during unauthorised recordings, but instead of placing the pulse signal within the back porch during the horizontal synchronisation intervals between picture information within a field, which would conflict with the black-level clamping operation of the television monitor or receiver immediately prior to displaying the picture information portion of a line, pulse signals were inserted into only those back porch intervals of the synchronisation pulses, in clusters thereof, during the vertical blanking interval.
A limitation of this technique is that the signals can not be inserted into the horizontal blanking intervals occurring during the scanning of lines of picture information within a field or frame, or any significant number of these. Thus the pulse signal could only be inserted during the vertical blanking interval at the end of a frame and at the start of the next frame, so as not to unduly deplete the picture image during normal playing of the treated signal.
In the Macrovision technique, however, the reason for the depletion of the picture image during normal playing of the treated signal, was not due to the black-level clamping problem experienced in the Sony technique. Rather, in the Macrovision technique, the pulse signals are specifically inserted into the colour burst portion of the horizontal blanking interval which affects the brightness of the displayed picture. Thus, the resultant technique if used in too many lines of the video signal during a frame, would cause a continuous variation in the brightness of the viewed picture which would upset normal viewing, as opposed to only introducing this effect on copied versions of the treated signal.
One of the reasons for inserting the pulse signal in the colour burst portion was to avoid interfering with the remainder of the back porch as this part of the signal during the vertical blanking interval is used by teletext and data casting systems as well.
Other methods which have similarly tried to prevent the acceptable copying of video tapes produce a phenomenon known as black-level depression. In these methods, the treated video signal""s colours which are meant to be grey are represented as black. The treatment of the video signal using this technique is relatively expensive and video tapes employing the same are expensive to purchase and typically require studio quality video recorders to make the original recording. Studio quality video recorders are considerably more expensive than home video recorders or modified home recorders and hence the use of this system is unattractive to video shop outlets who may be permitted to make original recordings in most instances.
It is an object of the present invention to provide a method and apparatus for treating a video signal for reproduction on a main or master video cassette tape using a modified standard VCR or professional VCR with AGC switched off, whereby the treated video signal substantially prevents or at least mitigates the ability to acceptably copy a video signal therefrom for displaying using a standard VCR having an AGC for adjusting the video signal level during recording but not during play.
It is a further object of the present invention to provide a method and apparatus for treating a video system and decoding the same which overcomes some of the deficiencies or limitations associated with the techniques described in the aforementioned prior art.
In accordance with one aspect of the present invention, there is provided a method for treating a video signal to mitigate the ability of a video signal recording system having an automatic gain control for adjusting the video signal level, copying an acceptable video signal for displaying therefrom, including:
inserting a pulse signal into the video signal during a horizontal blanking interval of a line of the video signal, said pulse signal having a magnitude sufficient to cause the treated video signal to overcompensate the automatic gain control of the video signal recording system; and
biasing a portion of the video signal during said horizontal blanking interval to offset the direct current (dc) voltage shift caused in the video signal by said inserting;
wherein said pulse signal is inserted into a sufficient number of lines of the video signal to cause visual impairment of the reproduced picture information contained in a copy made of the treated video signal by said video signal recording system.
In accordance with another aspect of the present invention, there is provided a method for decoding a treated video signal as defined in the preceding aspect of the present invention, including:
clamping or switching out the dc voltages of the inserted pulse signals and opposing pulse signals in the treated video signal to the video blanking level; and
stabilising the amplitude of the synchronisation pulses.
In accordance with a further aspect of the present invention, there is provided an apparatus for treating a video signal to mitigate the ability of a video signal recording system, having an automatic gain control for adjusting the video signal level, copying an acceptable video signal for displaying therefrom, including:
receiving means to receive the video signal;
pulse generating means to generate a pulse signal of a prescribed magnitude sufficient to cause the treated video signal to overcompensate the automatic gain control of the video signal recording system when inserted into the video signal;
inserting means to insert said pulse signal into the video signal during a horizontal blanking interval of a line of the video signal; and
biasing means to bias a portion of the video signal during said horizontal blanking interval to offset the dc voltage shift caused in the video signal by said pulse signal;
wherein said inserting means is controlled to insert said pulse signal into a sufficient number of lines of the video signal to cause visual impairment of the reproduced picture information contained in a copy made of the treated video signal by said video recording system.
In accordance with another aspect of the present invention, there is provided an apparatus for decoding a video signal treated in accordance with a method as defined in the first aspect of the present invention or an apparatus as defined in the preceding aspect of the present invention, including:
decoder receiving means to receive the treated video signal;
decoder clamping means to clamp or switch out the dc voltages of the inserted pulse signals and opposing pulse signals in the treated video signal to the video blanking level; and
stabilising means to stabilise the amplitude of the synchronisation pulses.